Precision measurements of beauty decays and the W boson mass at LHCb

Lead Research Organisation: University of Warwick
Department Name: Physics


In elementary particle physics we study the basic building blocks of Nature including those that have not naturally existed since the earliest stages of the Universe. In our Standard Model theory (SM) there are 12 fundamental matter particles, each with its antimatter twin. They interact by exchanging one of four types of "bosons". Over 40 years, this elegant theory has proven remarkably accurate in explaining what we see in our experiments. However the SM is not the final word. For example it fails to explain why the present day Universe contains any matter at all. The Big Bang should have created matter and antimatter in equal quantities, but we know that matter and antimatter "annihilate" when they meet, producing photons (one of the four types of bosons). Almost 14 billion years later, all that should remain is the annihilation photons. That is very nearly true, except that our Universe appears to possess one matter particle for every few billion photons. This seemingly insignificant imbalance is what makes up everything from the the stars and galaxies, to you and I. The SM simply cannot explain it. This is just one example of why we are certain of "new physics" beyond the SM.

The Large Hadron Collider is built to search for heavy new particles that are expected to exist. It does so by smashing protons (a type of "hadron") together at high energy. Bunches of 100 billion protons collide 30 million times each second. The energy is critical because Einstein's famous equation (E=mc^2) tells us that energy can be transformed into mass. The new particles are expected to be massive (heavy), so according to the equation we need large energy to produce them. After its first run in 2010-2012, the LHC has just begun a new journey into the unknown, with a three year "Run-II" at almost double the energy.

Of the four main LHC experiments, LHCb has a cunning strategy to look for new physics. Rather than search for the direct production of new particles it makes extremely precise measurements of "beauty hadrons". They have been known for decades, yet are of great interest because their behaviour can be indirectly affected by new particles. The theory of Quantum Mechanics allows particles to flicker in and out of existence in so called "loops", and the b-hadrons exhibit various phenomena that depend on them. The goal is to see the effects of loops containing new particles. We must study huge quantities of b-hadrons to discern tiny differences compared to the calculations of the SM theory. LHCb sees some million-billion of them each year. With the three years of Run-II data, I will make two measurements related to differences between matter and antimatter versions of b-hadrons. A key challenge will be to avoid being fooled by fake effects due to imperfections in the apparatus.

Quantum loops also affect the masses of the force carrying bosons. The mass of the W-boson is notoriously difficult to measure. While an uncertainty of 2 parts in 10,000 might seem impressive, a further reduction could reveal a deviation from the expectation of the SM. Two of the LHC experiments have already set out on a mission to do this, but their ultimate precision will be limited by how well we understand the details of proton collisions. I have shown that LHCb, which wasn't designed for this purpose, can actually make a similarly precise measurement and its special features will crucially reduce our dependence on how well we understand proton collisions.

One of the biggest challenges in all LHC experiments is to decide, within less than a second, which collisions to save for further study. More than 99.9% of them need to be discarded. This is the task of the "trigger" system and I have long been involved with the LHCb trigger and I will continue my prominent role. This is a challenging but exciting programme of research with the potential for great rewards - deviations from the SM predictions that would point us to new physics.


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Related Projects

Project Reference Relationship Related To Start End Award Value
ST/N004892/1 31/03/2017 31/08/2018 £485,990
ST/N004892/2 Transfer ST/N004892/1 01/09/2018 31/03/2022 £349,408
Description Standard model Precision Electroweak tests at Acute Rapidities
Amount € 1,875,781 (EUR)
Funding ID 865469 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 06/2020 
End 06/2025